The present invention pertains to a cylinder head of an internal combustion engine with at least one spark plug tube per cylinder, wherein the spark plug tube has an end area facing the combustion chamber of the engine with a connection thread for screwing a spark plug into the said cylinder head and an adjoining area with increased diameter, which area is open toward the outside of the engine.
The cylinder head of internal combustion engines seals the working space of the cylinder and transmits part of the thermal energy generated during the combustion to the coolant circulating in the cylinder head in order to thus limit the temperatures of the components as a whole.
The cylinder head is subject to mechanical stresses due to pretensioning and gas forces and to thermal load due to the combustion. The pretensioning forces of the cylinder head originate from the screw connection with the cylinder block, while the gas forces are caused by the combustion in the combustion chamber, which are thus also responsible for the thermal heating of the components.
In addition, the cylinder head is responsible for a considerable portion of the gas exchange of the combustion process and accommodates the intake and exhaust valves and, in gasoline engines or gas-operated diesel engines, the spark plug of every cylinder.
High-performance engines of modern design now usually have a plurality of intake and exhaust valves. The valves, which comprise a valve head and a valve shaft each, are sealingly in contact with a valve seat, which is milled into the cylinder head. The respective intake and exhaust channels of the engine are arranged downstream of the valve head.
To prevent leaks of the valve seat, which are caused by a one-sided wear of valve areas subject to increased stress, the valves are often arranged rotatably on engines of prior-art designs, so that they perform a small rotary movement, which usually takes place during the opening of the valve, during each stroke.
In addition, it may happen for various reasons that the valves do not seal one hundred percent, because, e.g., the central axis of the valve is located outside the central axis of the valve seat ring as a consequence of existing inaccuracies in manufacturing or installation, i.e., there is an offset or the valve seat is damaged.
Regrinding or milling of the valve seats in the cylinder head is necessary in case of greater wear or damage, such as scars or impacted areas. In extreme cases, the valve as well as the valve seat ring must be replaced. This is not only expensive, but also complicated.
Another problem arises for proper valve seating from the fact that current cylinder heads of internal combustion engines are made mostly of aluminum. This material shows an increased tendency to nonuniform material expansions, so that the cylinder head and, last but not least, the valve seat rings are warped along with it. This problem arises especially after a cold start of the engine. In particular, the parts of the cylinder head in the combustion chamber are now subject to high thermal loads.
Another of the possible causes of this undesired deformation of the valve seat rings can be seen in the spark plug tube present in the cylinder head. The spark plug tube is a mounting opening for the spark plug of the engine and is therefore needed on each cylinder head in which a spark plug is present. The spark plug tube may consist of a material different from that of the cylinder head, but in most cases it is milled directly into the cylinder head during the manufacture of the cylinder head in the form of a hole or recess. It makes it possible to accommodate the spark plugs and to make same accessible, and it must therefore also have an opening on one side.
The valve seat rings are inevitably arranged in the immediate vicinity of the thread of the spark plug because of the short distances between the valves. The individual assembly groups are proportional and show no warping in the cold state of the cylinder head. Consequently, they are 100% dimensionally accurate. During heating after the start-up of the internal combustion engine, the material of the cylinder head is subject to locally greatly different expansions. This in turn causes a nonuniform deformation of the valve seat rings as well, and it is consequently the cause of the so-called xe2x80x9ccold stall effect.xe2x80x9d
The valves become leaky, and the problem of depositions in the area of the leaky areas leads to accelerated wear of the valves and also of the valve seat ring. Even though the above-mentioned effect is equalized during uniform heating, e.g., longer operating time of the engine, the increased wear phenomena occurring in the valve seat area during the cold start phase may lead, on the whole, to premature failure of the engine.
The primary technical object of the present invention is to develop a cylinder head in which nonuniform deformation of the valve seat rings is extensively avoided. In addition, a process shall be provided that makes possible the manufacture of such a cylinder head.
To accomplish the technical object, it is proposed that on a cylinder head of an internal combustion engine with at least one spark plug tube per cylinder, in which the spark plug tube has an end area facing the combustion chamber of the engine with a connection thread for screwing in a spark plug in the cylinder head and an adjoining area with increased diameter, which said area is open toward the outside of the engine. The spark plug tube is designed such that it is cut through at right angles to its longitudinal direction.
Due to the cutting through at right angles to the longitudinal direction, the expansion of the cylinder head does not affect the spark plug tube, so that deformation of the valve seats is thus also avoided.
Using the solution being proposed here, it is now possible, for the first time ever, to equip both new cylinder heads and engines already in operation with such devices and thus to guarantee satisfactory operation both during the cold start phase and the hot running of the engine. The valve seats are not deformed any more, and the engine is subject to wear in considerably longer lifecycles only. A very inexpensive variant of the processing of old engines and of the improvement of new engine systems has thus been made possible.
Thus, it is also proposed according to the invention being presented here that a sleeve be inserted in the spark plug tube and that this sleeve be fastened in the spark plug tube by means of a permanently elastic sealant. The permanently elastic sealant makes possible the expansion of the cylinder head and of the spark plug tube to a limited extent without the components mutually affecting one another. The permanently elastic sealant, which may be manufactured, e.g., on the basis of a thermally loadable silicone, is thus used to compensate the different deformations of the corresponding cylinder head areas as a consequence of their different thermal expansions. At the same time, the sleeve seals the spark plug tube cut through against the escape of coolant in conjunction with the permanently elastic sealant if a coolant-carrying cavity of the engine block is touched during the cutting through.
Since the cross section of the spark plug tube must be increased for the sleeve to be able to be inserted into it, it is proposed in another embodiment of the present invention that the sleeve be provided with a length that corresponds to the length of the spark plug tube, so that the difference in diameters is again compensated and there is consequently no reduction in cross section in this sensitive area of the cylinder head, which could possibly lead to destruction of the cylinder head.
The process according to the present invention comprises the process steps described below. According to this process, the internal diameter of the spark plug tube is first increased by a material-removing process to a diameter that is greater than the external diameter of the sleeve. Machining processes as they are known per se may be used as the material-removing processes. The spark plug tube is subsequently cut through at right angles to its longitudinal axis, which is followed by the insertion of the sleeve into the spark plug tube, the sleeve being provided with the permanently elastic sealant. This permanently elastic sealant also has the advantage that the tightness of the cylinder head continues to be guaranteed as before, i.e., no coolant can escape.
The process according to the present invention as well as a cylinder head according to the solution being presented can be integrated in the motor vehicle manufacturing cycles without problems, without any greater changes being necessary in the course of the manufacture. In addition, the retrofitting of old cylinder heads is possible.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.